Frequency synthesizers are commonly implemented within wireless communication devices that transmit and receive encoded radio frequency (RF) signals. A number of different wireless communication techniques have been developed including frequency division multiple access (FDMA), time division multiple access (TDMA) and various spread spectrum techniques. One common spread spectrum technique used in wireless communication is code division multiple access (CDMA) signal modulation in which multiple communications are simultaneously transmitted over a spread spectrum radio-frequency (RF) signal. Some example wireless communication devices that have incorporated one or more wireless communication techniques include cellular radiotelephones, PCMCIA cards incorporated within portable computers, personal digital assistants (PDAs) equipped with wireless communication capabilities, and the like.
Frequency synthesizers of wireless communication devices may be used during both RF signal reception and RF signal transmission. For example, during RF signal reception of CDMA encoded signals, RF signals are typically mixed down to baseband signals, which can be converted to digital values. During the mixing down process, reference waveforms are produced by a frequency synthesizer that utilizes a local clock of the wireless communication device as a timing reference. After mixing the RF signal down to baseband, the baseband signals are typically passed through an analog-to-digital (A/D) converter to produce the digital values that can be tracked and demodulated. For example, a RAKE receiver can be used to track and demodulate multi-path signals of a CDMA system. A number of different CDMA architectures have been developed, such as for example, a heterodyne architecture that includes both an intermediate frequency (IF) section and an RF section, and a Zero IF architecture which converts incoming RF signals directly into baseband signals without first converting the RF signals to IF signals. Depending on the architecture, any number of frequency synthesizers may be implemented to provide reference waveforms to the mixers.
Frequency synthesizers are also used during RF signal transmission. In that case, baseband signals are up-mixed to RF. During the up-mixing process, the frequency synthesizer produces carrier RF waveforms. The carrier waveforms are then encoded with the baseband signal before being wirelessly transmitted. Again, the frequency synthesizer typically uses the local clock of the wireless communication device as the timing reference. For example, the carrier RF waveform may be created by a voltage controlled oscillator (VCO) whose frequency is determined by a phase locked loop (PLL). The timing reference for the PLL is a high precision low frequency crystal oscillator, such as a voltage controlled temperature compensated crystal oscillator (VCTCXO). The VCO may be off-chip, or alternatively integrated on-chip. The phase locked loop (PLL) that provides closed-loop analog control of the oscillator can either be integrated on the same chip as the VCO, or can likewise be a separate off-chip component.
Frequency variation of the VCO is a major concern. Frequency variation can be caused by one or more of a number of factors, including manufacturing variations, process variations, and frequency variations caused by changes in ambient conditions such as temperature. The analog voltage applied at the VCO can be modified to account for frequency errors, but the analog gain of the VCO is limited. Moreover, it is often desirable to reduce the gain of the VCO in order to limit the amount of noise introduced into the system. Frequency variation can be particularly problematic in integrated frequency synthesizers that integrate the VCO on-chip with the phase locked loop. In particular, varactors of an on-chip VCO may present more variation in mean capacitance than the VCO is able to compensate in voltage controlled capacitance.